Recently I have discovered the pipe to my radiator is looking a bit weird. From what I can tell, it does not quite look like corrosion, neither like mold. I think I recall there was some green paint on the pipe, but I am not sure. Neither would I know how it would turn into this. Any ideas what this could be?
You're seeing corrosion salts forming. In the presence of a water leak, copper will display greenish or bluish crust as the corrosion mineral in the water reacts and dries. Blue-green is copper sulfate, green is copper chloride/oxide. Fix the leak.
Discuss with your HVAC expert to find if you need to demineralize/deacidify the water in your heating system.
This is heated pipe, cold water pipes can show a greenish tinge where a lot of condensation from damp air coming in contact with the pipes condenses and drips off the copper pipe.
It's algae, looks like you have a leak. Algae needs a lot of water and sunlight to survive. You can kill it by cutting off either of those two elements. Then again, you could let it live, give it a name and have pet algae instead.
What is this? A biology test?
You can prove its algae by getting a dish of water, putting it on a window sill, putting some rocks in and then adding the green stuff to the rocks. If it grows, its algae.